Connecting Rod Bearing Failure Analysis

Connecting rod big-end bearings are precision-insert bearings. The insert-type bearing is usually not adjustable. However, it can be replaced, if the rod, crankpin, and other engine components are in good condition. When a rod bearing falls, an analysis should be made to determine the cause. Then
 the cause can be eliminated so that the failure will not repeated. Lets move into the failure analysis of con rod bearings.

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Analysis of Rod Bearing Failure

 Bearing Failure due to lack of Oil

When insufficient oil flows to a bearing, actual metal to metal contact results. The bearing overheats, and the bearing metal melts or is wiped out of the bearing shell.Welds may form between the rotating journal and bearing shell. There is a chance that the engine will " throw a rod". This means the rod will "freeze" to the crankpin and break, and parts of the rod will punch a hole through the engine block. Oil starvation of a bearing could result from clogged oil lines, a defective oil pump or pressure regulator, or insufficient oil in  the crankcase. Also, bearings with excessive clearance may pass all the oil from the pump, so other bearings are starved and will fail.

Fatigue failure of bearing

Repeated application of loads on a bearing fatigue the bearing metal. It starts to crack and flake out. Craters, or pockets, form in the bearing. As more and more of the metal is lost, the remainder carries a greater load and fatigues at a faster rate. Then complete bearing failure occurs.

Fatigue failure seldom occurs under average opening conditions. However certain conditions will cause this type of failure. For example, if a journal is worn out of round, the bearing will be overstressed with every crankshaft revolution. Also, if the engine is idled or operated at a low speed most of the time , the center part of the upper rod-bearing half will carry most of the load and will "fatigue out". If the engine is "lugged" by operating at maximum torque with wide open throttle, then most of all of the upper bearing half will fatigue out. High speed operation tends to cause fatigue failure of the lower bearing half.

Bearing Scratched by dirt in the Oil

Embeddability enabled a bearing to protect itself by allowing particles to embed in the bearing. Then they will not gouge out bearing material or scratch the rotating journal. Figure shows what happens when a particle embeds. The metal is pushed up around the particle, reducing oil clearance in the area. Usually the metal can flow outward enough to restore adequate oil clearance. However, if the dirt particles are too large they do not embed completely. They are carried with the rotating journal, gouging out scratches in the bearing. Also, if the oil is very dirty, the bearing becomes overloaded with particles. In either case, bearing failure soon occurs. 

Bearing Failure due to taper journal 

If the journal in tapered, one side of the bearing carries most or all of the load. This side will overheat and lose its bearing metal. With a tapered journal, both bearing halves fail on the same side. With a bent rod, failure will be on opposite sides.

Bearing failure from radius ride 

If the journal to crank cheek radius is not cut way sufficiently when the crankshaft is machined, the edge of the bearing ride on the radius. This causes cramming of the bearing, possibly poor seating, rapis fatigue and early failure. Radius ride is most likely to occur on a reground crankshaft.

Bearing failure from improper seating

Improper seating of bearing shell in the bore causes local high spots where oil clearances are too small. Figure shows what happens when particles of dirt are trapped between the bearing shell and the bearing bore, This reduces oil clearances (As at X). Also an air space exists which prevents proper cooling of the bearing (A). The combination can lead to premature bearing failure. So care should be taken in connecting rod bearing installation and replacement.

Bearing failure from Ridging 

Crankpin ridging or camming may cause failure of a partial oil groove type of replacement bearing installed without removal of the ridge. The ridge forms on the crankpin because of uneven wear between the part of the crankpin in contact with the partial oil groove and the part that runs on the solid bearing. The original bearing wears around the ridge. However, when a new bearing is installed, the center zone may be overloaded (at the ridge) and may son fail. A ridge so slight that it can be enough to cause failure. Failures of this sort have been reported in engines having ridges of less than 0.001 inch (0.025 mm). 

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